Conventionally, as applications of optical measurement technology of physical quantities using a light receiving device and a light emitting device, ranging sensors for measuring a distance to a detection target object and gas concentration measuring apparatuses for measuring the concentration of measurement target gas in the atmosphere have been proposed.
In particular, as gas concentration measuring apparatuses, non-dispersive infrared gas concentration measuring apparatuses for, using a feature that an absorption wavelength of a gas differs from gas to gas in a mid-infrared region, measuring gas concentration by detecting the amount of the absorption by a gas have been known. Gas concentration measuring apparatuses using this principle include, for example, an apparatus configured to, by combining a filter (transmission member) through which infrared rays limited to wavelengths at which measurement target gas has absorption characteristics are transmitted and an infrared sensor and measuring the amount of absorption of the infrared rays, measure concentration of the gas (see, for example, JP H09-33431 A).